The present invention relates to a method of machining a sleeve bearing which comprises a radial bearing portion of a dynamic pressure type with grooves (dynamic pressure grooves) formed on an inner peripheral surface thereof and, more particularly, to a method of machining a sleeve bearing of high accuracy used for a rotary section in a VTR or FDD (floppy disk drive).
A conventional method of machining a sleeve bearing of this type comprises sleeve machining steps shown in FIG. 3b, wherein grooves 13 are formed, by a tool 20, on an inner peripheral surface 12 of a cylindrical blank 11 which has been subjected to lathe-turning machining, as shown in (a) of FIG. 1. A plurality of small-diameter steel balls 22 are securely fixed on an outer peripheral portion of the groove forming tool 20 in diametrically opposite relationship such that a distance 25 between opposite two of the small-diameter steel balls 22 has a value corresponding to a depth of the grooves formed on the inner peripheral surface 12 of the blank 11, as shown in (b) of FIG. 1. At the time of such groove forming, the small-diameter steel balls 22 of the groove forming tool 20 not only form the grooves 13 on the inner peripheral surface 12 but also raise portions of the blank 11 on both side edges of the grooves 13. A steel ball 35 having an outer diameter which is several .mu.m to ten or more .mu.m larger than a diameter of the finished inner peripheral surface is passed through the blank 11 so as to remove the raised deformations generated on the side edges of the grooves 13 on the inner peripheral surface 12 and to finish the inner peripheral surface ((c) of FIG. 1: ball sizing). At the same time, the roughness of the inner peripheral surface 12 of the blank 11 which has been subjected to lathe-turning machining is improved by plastic working when the steel ball 35 is passed. Another conventional technique concerning a spiral groove bearing wherein an insert member with a spiral groove formed on an inner peripheral surface thereof is used for an FDD apparatus is disclosed in Japanese Patent Unexamined Publication No 63-311620.
In the above-described conventional method of machining the sleeve bearing, however, the steel ball 35 is passed i.e. when (ball sizing is performed) for removal of raised deformations after the groove forming of the inner peripheral surface 12, and for finish machining. Consequently, in the case of the sleeve bearing which is thin, if it is to be finished at once by passing the steel ball 35 for both of these purposes, there is a problem that the shape accuracy of the bearing portion will be deteriorated. Although the surface roughness after ball sizing will be described in detail later, there is another problem that the roughness of the finished surface (FIG. 11b) will be only about half of that of the inner peripheral surface 12 of the blank 11 (FIG. 11a), to thereby affect the durability of the sleeve bearing.
Thus, the conventional sleeve bearing machining method having such problems can not realize a reduction in the size and weight of the rotary section by shortening and thinning the sleeve bearing.